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Does a permanent magnet motor need a frequency converter?

1. Definition of the inverter:

The frequency converter is a power control device that uses frequency conversion technology and microelectronics technology to control the AC motor by changing the frequency of the motor’s working power supply.

Does a permanent magnet motor need a frequency converter?插图

The inverter adjusts the voltage and frequency of the output power supply by switching on and off the internal IGBT, and provides the required power supply voltage according to the actual needs of the motor, thereby achieving the purpose of energy saving and speed regulation. In addition, the inverter also has many protection functions, such as overcurrent, overvoltage, overload protection, etc. 

2. The function of the inverter:

Frequency conversion energy saving is mainly manifested in the application of fans and pumps, but not all occasions will be applicable (pay attention to the use occasions and conditions of use).

Power factor compensation saves energy. Reactive power not only increases line loss and equipment heating but more importantly, the reduction of power factor leads to a reduction in the active power of the grid. A large amount of reactive power is consumed in the line, resulting in low equipment use efficiency and serious waste. After using the frequency conversion speed control device, due to the function of the filter capacitor inside the frequency converter, the reactive power loss is reduced and the active power of the grid is increased.

Soft start saves energy, the hard start of the motor has a serious impact on the power grid, and it also requires too much power grid capacity. After using the variable frequency energy-saving device, the use of the soft start function of the frequency converter will make the starting current start from zero, and the maximum value does not exceed the rated current reducing the impact on the power grid and the requirements for power supply capacity and prolongs the service life of equipment and valves.

Does a permanent magnet motor need a frequency converter?插图1

3. What are the benefits of a permanent magnet motor with a frequency converter?

The advantages of the permanent magnet motor with the frequency converter mainly include the following aspects:

1. Play an optimal energy-saving effect: The permanent magnet motor can be adjusted by a frequency converter to achieve an optimal operation effect without extra work.

2. Overvoltage protection: The output of the inverter has a voltage detection function, and the inverter can automatically adjust the output voltage so that the motor does not withstand overvoltage. Even when the output voltage adjustment fails and the output voltage exceeds 110% of the normal voltage, the inverter will protect the motor by shutting down.

3. Under-voltage protection: When the voltage of the motor is lower than 90% of the normal voltage, the inverter will stop for protection.

4. Overcurrent protection: When the current of the motor exceeds 150%/ 3 seconds of the rated value or 200%/ 10 microseconds of the rated current, the inverter protects the motor by stopping.

5. Phase loss protection: monitor the output voltage, when the output phase is missing, the inverter will alarm, and the inverter will stop to protect the motor after a period of time.

6. Reverse phase protection: The inverter can be set so that the motor can only rotate in one direction, and the rotation direction cannot be set. Unless the user changes the phase sequence of the motor A, B, and C wiring, there is no possibility of a reverse phase.

7. Overload protection: The inverter monitors the motor current. When the motor current exceeds 120% of the rated current for 1 minute, the inverter protects the motor by stopping.

8. Grounding protection: The inverter is equipped with a special grounding protection circuit, which is generally composed of grounding protection transformers and relays. When one or two phases are grounded, the inverter will alarm. Of course, if the user requests, we can also design to protect the shutdown immediately after grounding.

9. Short-circuit protection: After the inverter output is short-circuited, it will inevitably cause overcurrent, and the inverter will stop protecting the motor within 10 microseconds.

10. Overclocking protection: The inverter has the maximum and minimum frequency limit function so that the output frequency can only be within the specified range, thus realizing the overclocking protection function.

11. Stall protection: Stall protection is generally aimed at synchronous motors. For an asynchronous motor, the stall during acceleration must be manifested as overcurrent, and the inverter realizes this protection function through overcurrent and overload protection. Stalls during deceleration can be avoided by setting a safe deceleration time during commissioning.

4. What are the differences between Vector control and V/F control mode?

1. Vector control mode

The vector control principle is to imitate the control principle of the DC motor, according to the dynamic mathematical model of asynchronous motor, using a series of coordinate transformations to decompose the stator current vector into an excitation component and torque component, the torque current component and excitation component of the motor are controlled separately.

The decoupling of magnetic field and torque is achieved after the rotor magnetic field is oriented, so as to control the torque of the asynchronous motor and obtain the control performance of the asynchronous motor close to that of the other-excited DC motor.

The specific approach is to decompose the stator current vector of the asynchronous motor into the current component that generates the magnetic field (excitation current) and the current component that generates the torque (torque current), and control the amplitude and phase between the two components at the same time, that is, control the stator current vector, so this control method is called vector control method.

2. V/F control mode
Does a permanent magnet motor need a frequency converter?插图2

V/F control, that is, the ratio of inverter output frequency and the output voltage is a constant value or proportional. For example, if the output voltage is 380V at 50HZ, the output voltage is 190V at 25HZ. When the inverter adopts V/F control mode, it does not depend much on the motor parameters. V/f control is proposed in order to get the ideal torque-speed characteristics, based on the idea of changing the power frequency for speed regulation while ensuring the flux of the motor remains unchanged, and the general-purpose inverter basically adopts this control mode. V/f control inverter is very simple in structure, but this type of inverter uses open-loop control and cannot achieve high control performance, moreover, at low frequency, torque compensation is necessary to change the low-frequency torque characteristics.

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3、V/F this control method is mostly used for fans and pumps energy-saving inverters.

The proportional relationship between V and f is pre-determined by considering the motor characteristics.

4、The application of vector control is generally a more demanding transmission occasion.

For example, the required constant torque speed range indicator is high, and the range of constant power speed regulation is relatively wide. Moreover, vector control is different from V/F control, which can output 100% torque at low speed, while V/F control cannot work at low speed due to insufficient torque.

5.V/F control – to control speed for the purpose

Control characteristics control accuracy is not high, low speed, the torque is obviously small, commonly used in the inverter one drag multiple occasions.

Vector control – it has a speed closed loop.

That is, the actual speed is measured from the load side and compared with the given value, which can get higher precision speed control and also has the highest torque output at low speed.

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